The study was designed to initially determine levels of moderate-to-vigorous physical activity (MVPA) occurring during physical education in 11-12 year olds using appropriate objective methods. Subsequently, the potential of a pedagogical method; ‘teaching games for understanding’ to increase PA levels and self determined motivation during PE lessons, and habitual physical activity during leisure time was examined. The most reliable and valid PA measurement tool for the chosen age group was the RT3 ® triaxial accelerometer. PA levels during PE lessons fell short of the recommended 50% (20 minute) criterion, with children accumulating 16.4 ± 2.3 minutes (44.9 ± 5.6%) of mean MVPA during lesson time. Seven day habitual activity monitoring revealed that time spent in MVPA on a PE day was significantly higher (P <0.05) than on a weekend day. This study also highlighted that on non PE days the lack of PE-related activity was not compensated by engagement in other activity. An investigation into the effects of a 12 week TGfU pedagogical strategy on MVPA and elements of Self Determination Theory during PE lessons revealed that boys assigned to the intervention displayed significantly higher (P <0.01) levels of MVPA, and significantly higher levels of autonomy (P < 0.05) post-intervention versus the control group. In addition, a non significant trend for an increase in habitual PA for boys assigned to the intervention lessons was revealed. No significant differences were displayed in the constructs of the TPB pre-post intervention and no significant benefits of TGfU were noted for girls. The reported increases in MVPA and levels of autonomy during PE lessons in boys using a TGfU approach are novel and promising. However it is suggested that future research incorporates such strategies in a health-promoting PE environment in addition to the traditional skills-based activities. This may have potential in enhancing MVPA during PE in girls and boys, and may promote greater transference to habitual physical activity levels. The potential for self determined environments positively impacting upon motivation and intentions to be physically active both during and outside of PE lessons warrants further exploration but over longer time periods.